This invention relates to new and useful improvements in heat strips or panels which comprise a film of electroconductive particles sandwiched between protecting layers and connectable to a source of electrical power so that the entire panel developes heat which is then radiated through the protective panels.
An example of such material is illustrated and described in U.S. Pat. No.: 3,865,626 dated Feb. 11, 1975 and this patent is hereby made of record in this application.
This patent teaches the use of a high strength polyester film of low surface resistance which is produced by treating one or both surfaces of a stretched polyester film of high mechanical strength with a solvent or swelling agent capable of swelling or superficially dissolving the polyester film surface, applying electroconductive particles, preferably carbon black, to the surface in a concentration corresponding to the desired surface resistance, and then subjecting the film to a heat treatment. In the case of pigments which are inert to the solvents or swelling agents employed, such as carbon black, the procedure is advantageously such that the electroconductive particles are dispersed in the solvents or swelling agents used for superfically dissolving or swelling the surface of the polyester film, the concentration of the dispersion, relative to the proportion of pigment, ranging from 0.1 to 12.0% by weight, preferably from 0.5 to 5.0%. The quantity of carbon black to be employed per square meter of film surface ranges from 0.1 g/m.sup.2, preferably from 0.3 g/m.sup.2 to 3.0 g/m.sup.2. The dispersion is applied to the film surface and the film is then subjected to a heat treatment.